The invention relates to a system for illuminating an object and notably to a system requiring illumination using polarized light. The invention is notably applicable in liquid-crystal projection and backprojection systems.
Liquid crystal cells that allow light to be modulated for displaying images need to be illuminated by means of polarized light in order to operate. When using a source of unpolarized light, a means for using the two polarizations of the light must be provided in order to avoid losing 50% of the luminous power emitted by the illuminating source, which does not take account of the losses by absorption in the polarizers.
A system described in U.S. Pat. No. 5,784,181 is notably known that provides two prisms whose cross sections are right-angled triangles and which sandwich a holographic polarizing beam splitter between faces each containing one side of the right angle of the right-angled triangles. The other faces of the prisms containing the other side of the right angle of the right-angled triangle are situated on one and the same plane and are used as exit faces. A source of unpolarized light emits a beam that illuminates a face forming the hypotenuse of a prism in a direction parallel to the cross section of the prism. The light reaches the holographic splitter which transmits light with a first polarization to the exit face of a prism which contains a device for transforming this first polarization into a second polarization, whereas it reflects light with a second polarization to the exit face of the other prism which does not contain a polarization conversion device. Holographic devices are associated with the exit faces that allow the directions of the beams, which have the same polarization downstream of the exit faces, to be changed, so as to make the beams parallel in order that they illuminate a liquid crystal cell in one and the same direction.
However, in such a system, the holographic devices must be illuminated or “read” under a Brewster angle of incidence of around 45°. Hence, they have difficulty operating under an incidence of around 30°. This system therefore has the drawback, since it uses holographic devices, of requiring an illuminating beam with a low divergence. Furthermore, this system is only effective within a limited spectral band. Typically, each holographic device is only effective within one primary spectral band (red, green or blue) of visible light.
The document U.S. Pat. No. 5,716,122 describes a polarization recycling system also using two prisms separated by a polarizing beam splitter of the thin-film multilayer type (see columns 4 and 5-column 17, lines 42-49); according to this document, the incident beam is not necessarily only composed of parallel rays, but may include nonparallel components (column 19, lines 15-17); however, the angles of incidence on the polarizing beam splitter that are mentioned in this document are in the range between 45° and 38°, which represents an undesirable limitation when the incident beam has a wide divergence, notably higher than or equal to 5° on either side of the general direction of this beam.
Other documents, such as U.S. Pat. No. 4,575,191, U.S. Pat. No. 4,161,349, U.S. Pat. No. 3,987,299, describe systems known as Koesters systems which also provide two prisms whose cross sections are right-angled triangles and which here sandwich a color splitter between the faces each containing one side of the right angle of the right-angled triangles.
The aim of the invention is therefore to provide a solution to the aforementioned drawbacks.
The invention therefore relates to an illuminating device comprising an optical source emitting an unpolarized light beam. A polarizing beam splitter is included between first faces of a first and of a second transparent prism. These prisms each have a second exit face both situated within one and the same plane. The first faces and the second faces of each prism are perpendicular. The light beam penetrates into the first prism through a third face of this prism and reaches the polarizing beam splitter that transmits the light with a first polarization direction and that reflects the light with a second polarization direction. Furthermore, the polarizing beam splitter comprises a grid polarizer situated between the first faces of the first and of the second prism. In addition, the light transmitted by the polarizing beam splitter is transmitted to a third face of the second prism that reflects it toward said second exit face of the second prism, whereas the light reflected by the polarizing beam splitter is transmitted to said third face of the first prism that reflects it toward said second exit face of the first prism. Thanks to the use of a grid polarizer, the polarization can be effectively separated over a wide range of angles of incidence; this also allows the third face of the first prism to be illuminated by a beam originating from the source with a wide divergence, notably greater than or equal to 5° on either side of the average direction of the beam; preferably, this divergence nevertheless remains less than or equal to 10° on either side of the average direction of the beam.
Advantageously, the system comprises a polarization rotator device associated with one of said second exit faces of the prisms. This device then allows the same polarization direction to be obtained downstream of two exit faces.
Advantageously, said grid polarizer is formed on the first face of the first prism or on the first face of the second prism.
An air gap is advantageously provided between, on the one hand, the grid polarizer and said first face of the first or of the second prism on which it is formed and, on the other, the other first face of the second or of the first prism (2 or 1), respectively, situated facing it. This air gap can be used for cooling the system.
According to one embodiment of the system of the invention, the non-right angles of the prisms are substantially equal to 60° opposite the first faces and to 30° opposite the second faces, and the average direction of the light beam F1 is substantially perpendicular to the third face of the first prism as it penetrates into this prism. Such a geometrical configuration ensures that the average direction of the beam passing through the second exit face of the first prism and the average direction of the beam passing through the second exit face of the second prism are substantially parallel to one another and perpendicular to these second faces. This configuration is therefore designed so that the optical axes of the beams emerging from the exit faces are parallel. It should also be noted that, downstream of the splitter, by way of this geometric configuration, the optical paths of the two separate beams are then approximately identical, a fact which is very advantageous for the application of this device to the illumination of a projection system imager.
Preferably, the index of the material of the prisms is less than or equal to 1.5, which allows angles of incidence less than 60° on the grid polarizer to be obtained, even for a wide divergence of the beam.
Advantageously, the system may comprise a light integrating device having an entry face that is optically coupled to said second exit faces of the prisms and that, receiving the beams reflected by the third faces of the prisms, delivers a beam through an exit face whose illumination is substantially homogeneous over this face. For this purpose, a rod integrator is advantageously used.
Advantageously, the light beam emitted by the source is an uncollimated beam such that the envelopes of the beams received by the light integrator device are such that all of the beams penetrate into the integrator device.